1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus in which ultrasonic waves are transmitted into the subject to obtain received signals through receiving ultrasonic waves reflected within a subject, thereby displaying an image based on the received signals.
2. Description of the Related Art
Hitherto, there has been used an ultrasonic diagnostic apparatus in which ultrasonic waves are transmitted toward the subject, specially a living body, ultrasonic waves reflecting from a tissue within a living body are received to generate received signals, and tomographic image of the living body is displayed on the basis of the received signals, thereby facilitating a diagnostic of diseases of the viscus inner organ or the like in the living body. According to such an ultrasonic diagnostic apparatus, usually, there is also provided such a function that a blood flow distribution within the living body as the subject is evaluated on the basis of received signals obtained through a plurality of number of times of receiving and transmitting of the ultrasonic waves in the same direction within the subject. The blood flow distribution thus obtained is displayed on the basis of the received signals.
FIG. 15 is a typical illustration showing the state of operations of an ultrasonic diagnostic apparatus. FIG. 16 is a block diagram showing a schematic circuit construction of an ultrasonic diagnostic apparatus according to the related art. In the following figures, the same parts are denoted by the same reference numbers as those of FIGS. 15 and 16.
In FIG. 15, an operator 100 puts a tip portion 20a of a probe 20 on the subject 200. On the tip portion 20a, there is mounted a piezo-electric transducer 21 (FIG. 16). On the rear end of the probe 20 there is provided a connector 22 detachably connected with a main body 10 of the ultrasonic diagnostic apparatus. The connector 22 and the tip portion 20a of the probe 20 are connected to each other through a cable 23. As the cable 23, in order to avoid intermixing of noises, generally, there is adopted a coaxial cable.
When the operator 100 operates a handler on a panel 12 of the main body 10, a transmission circuit 13 of the main body 10 generates high voltage pulses, which are applied through a signal line 23a within the cable 23 of the probe 20 to the piezo-electric transducer 21. Upon receipt of the high voltage pulses, the piezo-electric transducer 21 transmits ultrasonic waves into the subject 200. The ultrasonic waves transmitted from the piezo-electric transducer 21 are reflected within the subject and returned to the piezo-electric transducer 21. The ultrasonic waves thus received by the piezo-electric transducer 21 are converted into electrical received signals. The received signals are fed through the signal line 23a of the cable 23 to a receive circuit 14 within the main body 10 so as to be suitably amplified. The thus amplified received signals are subjected to a suitable signal processing in a signal processing circuit 15 and then fed to a display unit 16 in which a tomographic image within the body of the subject 200 is displayed on a display screen 11 (FIG. 15).
In the signal processing circuit 15, there is evaluated a velocity of a blood flow on each portion within the tomographic plane on the basis of the received signal obtained through a plurality of number of times of transmitting and receiving in the same direction within the body of the subject 200. In the display unit 16, for example, a velocity distribution of the blood flow is color-displayed upon superposing it on the above-mentioned tomographic image.
Incidentally, a fundamental technology for obtaining tomographic images and a velocity distribution of the blood flow is well known. Also, with respect to the point that what circuit algorithm is used for evaluating the tomographic images and the velocity distribution of the blood flow, it is not essential matter for the present invention. Thus, additional explanation as to how the tomographic images and the velocity distribution of the blood flow are evaluated will be omitted.
It is noted that such states that the apparatus is adjusted in its entirety to obtain the tomographic images and the velocity distribution of the blood flow are referred to as "B-mode" and "Doppler mode", respectively.
As shown in FIG. 16, the signal line 23a of the cable 23 has a capacitance component 24, and the piezo-electric transducer 21 itself also has a capacitance component. Hence, there is such a problem that the received signals obtained in the piezo-electric transducer 21 will be remarkably attenuated owing to their capacitance components while transferred to the receive circuit 14. Specifically, the received signals obtained in the piezo-electric transducer 21 are high frequency of signals associated with the frequency of ultrasonic waves and thus are remarkable in attenuation. In addition, since such received signals are ones obtained through receipt of the ultrasonic waves reflected within the body of the subject 200 (FIG. 15), the received signals involved in the ultrasonic waves especially reflected on the depth portions within the body are extremely weak. Thus, the attenuation of the received signals has a large effect on the resolution of the images obtained and the like.
FIGS. 17, 18 and 19 are each a view useful for understanding a measure to be taken for preventing an attenuation of the received signals. In those figures, the same parts are denoted by the same reference numbers.
According to an example shown in FIG. 17, an inductive component (inductor) 31 is connected in parallel with the capacitance component 24 so as to constitute a resonance circuit 30. The resonance circuit 30 serves to prevent an attenuation of received signals having a frequency corresponding to that of the ultrasonic waves.
According to an example shown in FIG. 18, the inductor 31 is incorporated into the tip portion 20a of the probe 20 to constitute a resonance circuit 30 in the combination use with the capacitance component of the piezo-electric transducer. In addition, the receive circuit 14 is also incorporated into the tip portion 20a of the probe 20. In this case, an attenuation of the received signals due to the capacitance component of the piezo-electric transducer 21 can be prevented by the incorporation of the inductor 31. Further, since the capacitance component 24 of the cable 23 is located in the lower stage side of the receive circuit 14, there is no problem.
According to an example shown in FIG. 19, in comparison with that of FIG. 17, a resistance element 32 is connected in series with the inductor 31. The measures according to the examples shown in FIGS. 17 and 18 involve a high Q value of the resonance. Thus, there is such a fear that a ringing of the received signals continues for a long time and then time resolution (resolution in a depth direction within the subject) will be deteriorated. In view of this, generally, there is adopted a technique in which the resistance element 32 for dumping is connected as shown in FIG. 19 to suppress the Q value.
According to the related art as described above, it is possible to contribute to the improvement in sensitivity on a specific frequency on a fixing basis (prevention of an attenuation of the received signals). On the other hand, it should be noticed, however, that the improvement in a sensitivity and the time resolution are in a relation of the trade-off. In addition, in view of the fact that for example, in the B-mode, the improvement of the time resolution may bring a better image while making somewhat the sacrifice of the sensitivity. However, in the Doppler mode, it is desirable that the sensitivity is improved even making somewhat the sacrifice of the time resolution. Therefor, there is a case such that it is not preferable to adjust fixedly on a one way basis the resonance circuit. To satisfy these matters using the conventional technique, there is a need to prepare a number of probes which are adjusted for every use. This causes the cost to be increased and also the maintenance and the management to be troublesome.